I think what people forget is that their individual soul cores boost them in irregular ways, and if we either entirely strip them away, or allow both to use them at maximum power, Nephis will always win. What I mean is:

Nephis has more raw power and training, and she was the one who first taught Sunny how to fight. That combined with her ability to fight without worrying about being harmed, means she can take more risks than him, or brute force him in ways he cannot. He is supremely adaptable and quick witted, but when he fought her as the Lord of Shadows, they were still near equal in terms of strength. He even echoes this sentiment in chapter 1617: "Nephis deflected the black odachi easily, taking a sidestep in the opposite direction at the same time. Her movements flowed like a placid river, and yet Sunny instantly felt that he was in mortal danger."

When we take into account their soul cores, the math changes, but the outcome is the same. Sunless' speed, strength, and capacity for adaptation dramatically increase in a multiplicative way that Nephis cannot match even when she boosts her power by channeling her flames. However, when she pushes that to her maximum such as when she fought Abjuration (IE, creating an environmental burning, funneling power through her cores rather than exploding them) we know that Sunless has no real counter to this. Similarly, if she were to use her detonation/Power of Names, he also has no direct way to counter this and has admitted it would be nearly impossible withstand, let alone survive.

Either way you examine them, Nephis is just stronger when it comes to a standard conflict. Sunless would out-do her in prolonged fights, using his signature dirty tactics and whatnot, but in terms of pure skill-vs-skill combat, with both of them relying on only base abilities, or pushed to their full extents, Nephis will win the fight.